The present invention relates to a noise reduction technique in an imaging system utilizing an imaging element such as a CCD (charge coupled device) and more specifically to a technique for reducing noise generated by transmission of digital image data with a code conversion system, for example a technique which can effectively be adapted to an electronic still camera (a so-called digital camera) and a video camera.
As shown in FIG. 10, there is provided an imaging system such as an electronic still camera or a video camera in which an analog video signal outputted from a CCD 10 is converted into a digital signal in an LSI (large scale semiconductor integrated circuit) for AD conversion 20, this video signal is processed in a DSP (digital signal processor) 30 and the signal is then displayed on a display 80. The CCD 10 and DSP 30 are also packed respectively in a semiconductor integrated circuit like the LSI for AD conversion 20, and these semiconductor integrated circuits are mounted on a printed wiring substrate 100 to form an imaging system.
The inventors of the present invention have investigated in detail the cause of noise appearing on a display image of such an imaging system. As a result, the inventors have found the major causes in that power supply noise generated at the time of outputting video data from the AD conversion LSI 20 in order to transmit the AD-converted video data to the DSP 30 migrates into the CCD side via the power supply line (Vcc line and ground line) on the printed wiring substrate and thereafter appears on the video signal to be inputted to the AD conversion LSI, and that such power supply noise migrates into the input terminal side from the output circuit side through the power supply and semiconductor substrate within the AD conversion LSI.
Since an output circuit of LSI may drive a load such as external printed wiring which is larger than a load within a chip, an output element used is also large in size (10 times or more) in comparison with the elements forming the internal circuit, such as the AD conversion circuit, and therefore the circuit is usually designed to allow a comparatively large amount of current to flow. Therefore, it is thought that noise appears on the power supply because a large current flows when an output signal is changed over. Moreover, noise generated in the output circuit is also propagated to an internal circuit other than the input circuit via the substrate, but since the AD conversion LSI includes an amplifying circuit such as a PGA (programmable gain amplifier) for amplifying an input analog signal, the noise propagated to the input side is also amplified together with the video signal, and thereby display quality may be deteriorated.
Therefore, the inventors have attempted, in order to reduce noise resulting from operations of the output circuit, connection of a bypass capacitor having a comparatively large capacity to the power supply terminal of the AD conversion LSI as a measure to reduce the noise. However, it has been proved that providing a large bypass capacitor results in increase of a chip size and causes reduction of system loading efficiency and moreover, providing only the bypass capacitor cannot remove the noise sufficiently.
Therefore, the inventors have reached the conclusion that it is very effective for reduction of noise to reduce a through-current in the output circuit resulting from the change-over of digital video data outputted from the AD conversion LSI 20. One way to reduce the through-current in the output circuit is to lower the power supply voltage and lower a driving power of the output circuit. However, in this case, the transmission rate of the signal is lowered and thereby performance of the system can no longer be satisfied.
Therefore, it is an object of the present invention to provide a semiconductor integrated circuit which can reduce a through-current flowing into an output circuit when an output is changed over and thereby reduce noise appearing in the output circuit.
Another object of the present invention is to provide an imaging system which can improve image quality by reducing noise generated in an output circuit which converts an analog video signal outputted from a solid state imaging element such as CCD, into digital video data, and which then outputs such digital video data, without lowering the transmission rate of the signal.
The aforementioned and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.
An outline of the typical inventions disclosed in this specification will be explained as follows.
Namely, the AD-converted digital video data is encoded by a differential encoding method before it is outputted and such encoded signal is then outputted after conversion to gray code or conversion to a predetermined code through addition of a certain fixed value. In more practical terms, after an analog color video signal outputted from an imaging element is AD-converted in the AD conversion circuit, a difference between the codes of adjacent pixels is obtained in regard to the same color after the AD conversion and an output code of this difference process is then converted into a code with less change-over bits between the adjacent codes. Through such code conversion, the number of bits changing when the output digital signal is changed over is lowered, thereby a through-current in the output circuit is reduced and noise due to change of output can be lowered.
Moreover, the present invention comprises an amplifying circuit for amplifying an analog color video signal outputted from an imaging element, an AD conversion circuit for converting the amplified signal into a digital signal, a differential means for obtaining a difference of the codes of adjacent pixels in regard to the same color after the AC conversion, and a code conversion means for code conversion of an output of the differential means. Thereby, the number of bits changing when a digital signal output from a semiconductor integrated circuit changes is reduced, and thereby a through-current in an output circuit can also be reduced.
As the code converting means, it is preferable to use a binary gray code converting circuit for changing the input binary code into the gray code. Thereby, the number of bits which change when the output digital signal is changed over can surely be reduced.
Moreover, as the code converting means, it is also possible to use a circuit for adding a fixed value to the input code or subtracting a fixed value from the input code. Thereby, the number of bits changing when the output digital signal is changed over can be reduced considerably. The reason is based on the following principle. That is, the differential element of the digital video data is concentrated to the data near zero such as 0, +1, xe2x88x921 or the like. Therefore, the data becomes all xe2x80x9c0xe2x80x9d in the binary code, but since xe2x88x921 is all xe2x80x9c1xe2x80x9d, when the differential data changes to xe2x88x921 from 0 or vice versa, all bits change and thereby noise is generated. Therefore, generation of changes to all xe2x80x9c1xe2x80x9d from all xe2x80x9c0xe2x80x9d or vice versa is prevented by adding a fixed value to the input code or subtracting a fixed value from the input code.
Moreover, the differential means explained above is constructed by a delay circuit for delaying an output code of the AD conversion circuit and a subtraction means for obtaining the difference between the code delayed with the delay circuit and the input code, and the delay circuit changing the delay time depending on color arrangement of the input video signal. Thereby, even if the color arrangement of a filter used is different, it may be easily covered by changing a delay time of the delay circuit.
Moreover, the imaging system of the present invention is constructed by an imaging element being provided with a color filter, an amplifying circuit for amplifying an analog color video signal outputted from the imaging element, an AD conversion circuit for converting the amplified signal into a digital signal, a differential means for obtaining a difference between codes of adjacent pixels in regard to the same color after the AD conversion, a semiconductor integrated circuit being provided with a first code converting means for code conversion of an output of the differential means and a semiconductor integrated circuit for image processing being provided with a second code converting means for converting the code outputted from the semiconductor integrated circuit and an image processing circuit.
According to the means explained above, a through-current in the output circuit of the semiconductor integrated circuit provided with the AC conversion circuit can be reduced, and noise resulting from change of output can also be controlled, and as a result, display quality can be improved.
It is preferable that the first code conversion means uses a binary gray code conversion circuit for converting a binary code to the gray code, while the second code conversion means uses a gray binary code conversion circuit for converting the gray code to the binary code. Thereby, the number of bits which change whenever the output digital signal is changed over can be surely reduced and the display quality can be improved.
Moreover, it is also possible to use a circuit for adding a fixed value to the input code or subtracting a fixed value from the input code as the code conversion means explained above. Accordingly, the number of bits which change when the output digital signal is changed over can be reduced considerably and the display quality can also be improved.
Moreover, the differential means is constructed by a delay circuit for delaying an output code of the AD conversion circuit and a subtraction circuit for obtaining a difference between the code delayed by the delay circuit and the input code, while the delay circuit is constructed to change a delay time depending on the color arrangement of the input video signal. Thereby, even if the color arrangement of the filter used is different, it can be covered easily by changing a delay time of the delay circuit.
Moreover, in the construction of the present invention, a storage means is provided for storing digital video data, the semiconductor integrated circuit for image processing explained above is provided with a data compression circuit for compressing the code converted by the second code converting means and a data expanding circuit for expanding the compressed data and the data compressed by the data compression circuit is stored in the storage means explained above. Thereby, many video data can be stored in storage means of small storing capacity while video data compatibility is assured.